Editing Horizontal branch

{{Short description|Stage of stellar evolution}}
[[File:M5 colour magnitude diagram.png|thumb|right|upright=1.4|[[Hertzsprung–Russell diagram]] for [[globular cluster]] [[Messier 5|M5]], with the horizontal branch marked in yellow, RR Lyrae stars in green, and some of the more luminous [[red-giant branch]] stars in red

{{leftlegend|#0A88FF|[[Asymptotic giant branch]] (AGB)|outline=gray}} {{leftlegend|#FF420E|Upper [[red-giant branch]] (RGB)|outline=gray}} {{leftlegend|#FFD010|Horizontal branch (HB)|outline=gray}} {{leftlegend|#479406|[[RR Lyrae variable]] (RR)|outline=gray}} {{leftlegend|#7E0021|End of [[main sequence]], [[subgiant|subgiant branch]], and lower RGB|outline=gray}}]]
The '''horizontal branch''' ('''HB''') is a stage of [[stellar evolution]] that immediately follows the [[red-giant branch]] in stars whose masses are similar to the [[Sun]]'s. Horizontal-branch stars are powered by [[helium fusion]] in the core (via the triple-alpha process) and by [[hydrogen fusion]] (via the [[CNO cycle]]) in a shell surrounding the core. The onset of core helium fusion at the tip of the [[red-giant branch]] causes substantial changes in [[Star#Structure|stellar structure]], resulting in an overall reduction in [[Luminosity#In astronomy|luminosity]], some contraction of the stellar envelope, and the surface reaching higher temperatures.

==Discovery==
Horizontal branch stars were discovered with the first deep photographic [[Photometry (astronomy)|photometric]] studies of [[globular clusters]]<ref>
{{Citation
  | last1 = Arp  | first1 = H. C.
  | author1-link = Halton Arp
  | last2 = Baum | first2 = W. A.
  | last3 = Sandage | first3 = A. R.
  | author3-link = Allan Sandage
  | title = The HR diagrams for the globular clusters M 92 and M 3
  | journal = [[Astronomical Journal]]
  | volume = 57
  | pages = 4–5
  | year = 1952
  | doi = 10.1086/106674
| bibcode=1952AJ.....57....4A
}}</ref><ref>
{{Citation
  | last1 = Sandage | first1 = A. R.
  
  | title = The color-magnitude diagram for the globular cluster M 3
  | journal = [[Astronomical Journal]]
  | volume = 58
  | pages = 61–75
  | year = 1953
  | doi = 10.1086/106822
| bibcode=1953AJ.....58...61S
}}
</ref>
and were notable for being absent from all [[open clusters]] that had been studied up to that time. The horizontal branch is so named because in low-[[metallicity]] star collections like [[globular clusters]], HB stars lie along a roughly horizontal line in a [[Hertzsprung–Russell diagram]]. Because the stars of one globular cluster are all at essentially the same distance from us, their apparent magnitudes all have the same relationship to their absolute magnitudes, and thus absolute-magnitude-related properties are plainly visible on an H-R diagram confined to stars of that cluster, undiffused by distance and thence magnitude uncertainties.

==Evolution==
[[File:Evolutionary track 1m.svg|thumb|upright=1.2|The evolutionary track of a sun-like star, showing the horizontal branch and red clump region]]
After exhausting their core hydrogen, stars leave the [[main sequence]] and begin [[thermonuclear fusion|fusion]] in a hydrogen shell around the helium core and become [[giant star|giants]] on the [[red-giant branch]].  In stars with masses up to 2.3 times the mass of the [[Sun]] the helium core becomes a region of [[degenerate matter]] that does not contribute to the generation of [[energy]]. It continues to grow and increase in [[temperature]] as the [[hydrogen fusion]] in the shell contributes more [[helium]].<ref name=karttunen_oja2007/>

If the [[star]] has more than about 0.5 [[solar mass]]es,<ref>{{cite web|title=Post Main Sequence Stars|url=http://outreach.atnf.csiro.au/education/senior/astrophysics/stellarevolution_postmain.html|publisher=Australia Telescope Outreach and Education|access-date=2 December 2012}}</ref><!-- previously listed as 0.26 solar mass, but unable to find citation --> the core eventually reaches the temperature necessary for the [[Stellar nucleosynthesis|fusion]] of helium into carbon through the [[triple-alpha process]]. The initiation of [[helium fusion]] begins across the core region, which will cause an immediate temperature rise and a rapid increase in the rate of [[Stellar nucleosynthesis|fusion]].  Within a few seconds the core becomes non-[[Degenerate matter|degenerate]] and quickly expands, producing an event called [[helium flash]]. Non-degenerate cores initiate fusion more smoothly, without a flash.  The output of this event is absorbed by the layers of [[plasma (physics)|plasma]] above, so the effects are not seen from the exterior of the star. The star now changes to a new [[Hydrostatic equilibrium|equilibrium]] state, and its evolutionary path switches from the [[red-giant branch]] (RGB) onto the horizontal branch of the [[Hertzsprung–Russell diagram]].<ref name=karttunen_oja2007/>

Stars initially between about {{solar mass|2.3}} and {{solar mass|8}} have larger helium cores that do not become degenerate.  Instead their cores reach the [[Schönberg–Chandrasekhar limit|Schönberg–Chandrasekhar mass]] at which they are no longer in hydrostatic or thermal equilibrium.  They then contract and heat up, which triggers helium fusion before the core becomes degenerate.  These stars also become hotter during core helium fusion, but they have different core masses and hence different luminosities from HB stars.  They vary in temperature during core helium fusion and perform a [[blue loop]] before moving to the asymptotic giant branch.  Stars more massive than about {{solar mass|8}} also ignite their core helium smoothly, and also go on to burn heavier elements as a [[red supergiant]].<ref name=salaris>{{cite journal|bibcode=2005essp.book.....S|title=Evolution of Stars and Stellar Populations|url=https://archive.org/details/evolutionofstars0000sala|url-access=registration|journal=Evolution of Stars and Stellar Populations|pages=400|last1=Salaris|first1=Maurizio|last2=Cassisi|first2=Santi|year=2005}}</ref>

Stars remain on the horizontal branch for around 100 million years, becoming slowly more luminous in the same way that main sequence stars increase luminosity as the [[virial theorem]] shows.  When their core helium is eventually exhausted, they progress to helium shell burning on the [[asymptotic giant branch]] (AGB).  On the AGB they become cooler and much more luminous.<ref name=karttunen_oja2007/>

==Horizontal branch morphology==
Stars on the horizontal branch all have very similar core masses, following the helium flash.  This means that they have very similar luminosities, and on a [[Hertzsprung–Russell diagram]] plotted by visual magnitude the branch is horizontal.

The size and temperature of an HB star depends on the mass of the hydrogen envelope remaining around the helium core.  Stars with larger hydrogen envelopes are cooler.  This creates the spread of stars along the horizontal branch at constant luminosity.  The temperature variation effect is much stronger at lower [[metallicity]], so old clusters usually have more pronounced horizontal branches.<ref name="KippenhahnWeigert2012">{{cite book|author1=Rudolf Kippenhahn|author2=Alfred Weigert|author3=Achim Weiss|title=Stellar Structure and Evolution|url=https://books.google.com/books?id=wdSFB4B_pMUC&pg=PA408|date=31 October 2012|publisher=Springer Science & Business Media|isbn=978-3-642-30304-3|pages=408–}}</ref>

Although the horizontal branch is named because it consists largely of stars with approximately the same absolute magnitude across a range of temperatures, lying in a horizontal bar on a color–magnitude diagrams, the branch is far from horizontal at the blue end. The horizontal branch ends in a "blue tail" with hotter stars having lower luminosity, occasionally with a "blue hook" of extremely hot stars.  It is also not horizontal when plotted by bolometric luminosity, with hotter horizontal branch stars being less luminous than cooler ones.<ref name=yee>{{cite journal|bibcode=1994ApJ...423..248L|title=The Horizontal-Branch Stars in Globular Clusters. II. The Second Parameter Phenomenon|journal=The Astrophysical Journal|volume=423|pages=248|last1=Lee|first1=Young-Wook|last2=Demarque|first2=Pierre|last3=Zinn|first3=Robert|year=1994|doi=10.1086/173803|doi-access=free}}</ref>

The hottest horizontal-branch stars, referred to as extreme horizontal branch, have temperatures of 20,000–30,000&nbsp;K. This is far beyond what would be expected for a normal core helium burning star. Theories to explain these stars include binary interactions, and "late thermal pulses", where a thermal pulse that [[asymptotic giant branch]] (AGB) stars experience regularly, occurs after fusion has ceased and the star has entered the superwind phase.<ref>{{cite journal|doi=10.1088/2041-8205/737/2/L27|title=RAPIDLY PULSATING HOT SUBDWARFS IN ω CENTAURI: A NEW INSTABILITY STRIP ON THE EXTREME HORIZONTAL BRANCH?|journal=The Astrophysical Journal|volume=737|issue=2|pages=L27|year=2011|last1=Randall|first1=S. K.|last2=Calamida|first2=A.|last3=Fontaine|first3=G.|last4=Bono|first4=G.|last5=Brassard|first5=P.|bibcode=2011ApJ...737L..27R|doi-access=free}}</ref>  These stars are "born again" with unusual properties. Despite the bizarre-sounding process, this is expected to occur for 10% or more of post-AGB stars, although it is thought that only particularly late thermal pulses create extreme horizontal-branch stars, after the planetary nebular phase and when the central star is already cooling towards a white dwarf.<ref>{{cite journal|bibcode=2008ASPC..391....3J|title=Hydrogen-Deficient Stars: An Introduction|journal=Hydrogen-Deficient Stars|volume=391|pages=3|last1=Jeffery|first1=C. S.|year=2008}}</ref>

==The RR Lyrae gap==
[[Image:M3 color magnitude diagram.jpg|right|upright=1.2|thumb|[[Hertzsprung–Russell diagram]] for the globular cluster [[Messier 3|M3]]]]
[[Globular cluster]] CMDs ([[Hertzsprung–Russell diagram|Color-Magnitude diagram]]s) generally show horizontal branches that have a prominent gap in the HB.  This gap in the CMD incorrectly suggests that the [[Star cluster|cluster]] has no [[star]]s in this region of its CMD.   The gap occurs at the [[instability strip]], where many [[Variable star|pulsating stars]] are found. These pulsating horizontal-branch stars are known as [[RR Lyrae variable]] stars and they are obviously variable in [[brightness]] with periods of up to 1.2 days.<ref>{{Cite web
  | last = American Association of Variable Star Observers
  | author-link = AAVSO
  | title = Types of Variables
  | url = http://www.aavso.org/types-variables
  | access-date = 12 March 2011
  | archive-date = 17 October 2018
  | archive-url = https://web.archive.org/web/20181017170335/http://www.aavso.org/types-variables
  | url-status = dead
  }}</ref>
  
It requires an extended observing program to establish the star's true (that is, averaged over a full period) [[apparent magnitude]] and [[Star#Classification|color]]. Such a program is usually beyond the scope of an investigation of a cluster's color–magnitude diagram.  Because of this, while the [[variable stars]] are noted in tables of a cluster's stellar content from such an investigation, these [[variable stars]] are not included in the graphic presentation of the cluster CMD because data adequate to plot them correctly are unavailable.  This omission often results in the ''RR Lyrae gap'' seen in many published globular cluster CMDs.<ref name="Stevenson2015">{{cite book|author=David Stevenson|title=The Complex Lives of Star Clusters|url=https://books.google.com/books?id=nihACQAAQBAJ&pg=PA70|date=9 May 2015|publisher=Springer|isbn=978-3-319-14234-0|pages=70–}}</ref>

Different globular clusters often display different HB ''morphologies'', by which is meant that the relative proportions of HB stars existing on the hotter end of the RR Lyr gap, within the gap, and to the cooler end of the gap varies sharply from cluster to cluster. The underlying cause of different HB morphologies is a long-standing problem in [[stellar astrophysics]]. [[Star#Chemical composition|Chemical composition]] is one factor (usually in the sense that more metal-poor clusters have bluer HBs), but other stellar properties like [[Star#Age|age]], [[Star#Rotation|rotation]] and [[Star#Chemical composition|helium content]] have also been suggested as affecting HB [[morphology (astronomy)|morphology]]. This has sometimes been called the "Second Parameter Problem" for globular clusters, because there exist pairs of globular clusters which seem to have the same [[metallicity]] yet have very different HB morphologies; one such pair is [[NGC 288]] (which has a very blue HB) and [[NGC 362]] (which has a rather red HB). The label "second parameter" acknowledges that some unknown physical effect is responsible for HB morphology differences in clusters that seem otherwise identical.<ref name=yee/>

==Relationship to the red clump==
A related class of stars is the ''clump giants'', those belonging to the so-called [[red clump]], which are the relatively [[Star#Age|younger]] (and hence [[Star#Mass|more massive]]) and usually more [[Star#Chemical composition|metal-rich]] [[population I]] counterparts to HB stars (which belong to [[population II]]). Both HB stars and clump giants are fusing [[helium]] to [[carbon]] in their cores, but differences in the [[Star#Structure|structure]] of their outer layers result in the different types of stars having different radii, [[Effective temperature#Star|effective temperatures]], and [[Star#Classification|color]]. Since [[Star#Classification|color index]] is the horizontal coordinate in a [[Hertzsprung–Russell diagram]], the different types of star appear in different parts of the CMD despite their common [[energy]] source. In effect, the red clump represents one extreme of horizontal-branch morphology: all the stars are at the red end of the horizontal branch, and may be difficult to distinguish from stars ascending the red-giant branch for the first time.<ref name="KarttunenKröger2007">{{cite book|author1=Hannu Karttunen|author2=Pekka Kröger|author3=Heikki Oja|author4=Markku Poutanen|author5=Karl Johan Donner|title=Fundamental Astronomy|url=https://books.google.com/books?id=DjeVdb0sLEAC&pg=PA249|date=9 August 2007|publisher=Springer Science & Business Media|isbn=978-3-540-34144-4|pages=249–}}</ref>

==References==
{{Reflist|30em|refs=

<ref name=karttunen_oja2007>{{citation | first1=Hannu | last1=Karttunen | first2=Heikki | last2=Oja | title=Fundamental astronomy | edition=5th | publisher=Springer | year=2007 | isbn=978-3-540-34143-7 | page=249 | url=https://books.google.com/books?id=DjeVdb0sLEAC&pg=PA249 }}</ref>

}}

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{{DEFAULTSORT:Horizontal Branch}}
[[Category:Horizontal-branch stars|*]]
[[Category:Star types]]
[[Category:Concepts in stellar astronomy]]

[[cs:Hertzsprungův-Russellův diagram#Horizontální větev]]